W-Band Traveling-Wave Microstrip Comb-Line Antenna With Shaped Radiation Pattern for Intelligent Transportation Systems

In this article, a comb-line array (CLA) antenna with shaped patterns is proposed for millimeter-wave traffic monitoring radars in intelligent transportation systems (ITS). It is demonstrated that radiating elements with a wide tuning range of coupling coefficient, low cross-polarization, and independent control of amplitude and phase are necessary for a CLA to realize a shaped pattern. However, elements with these characteristics have not been reported. It is found that the rectangle patches with recessed microstrip-line feeds possess these characteristics simultaneously when working as radiating elements of a CLA. In addition, it is found for the first time that strong standing waves can persist in the feedline of a CLA even though the impedance matching at its input port is good. By introducing the matching element and reflection canceling slits, traveling-wave mode can be achieved in the feedline. Consequently, the amplitude and phase of each element of the proposed CLA can be flexibly controlled and a shaped pattern can be then generated. To confirm this design, a microstrip CLA with 16 radiating elements is designed and measured. It has a simple and compact structure and achieves gain of 14.3 dBi at 80 GHz. More importantly, its patterns agree well with the objective cosecant squared curve across the frequency range from 79 to 81 GHz. These features make the proposed antenna suitable for the Internet of Things-based ITS.